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United States Patent |
6,238,536
|
Lundgren
,   et al.
|
May 29, 2001
|
Arrangement for analysis of exhaust gases
Abstract
Exhaust gas analyzers are disclosed for analyzing the exhaust gases from a
combustion process including a sensor unit mounted for direct contact with
the exhaust gases in which the sensor unit includes a number of sensors
such as lambda sensors, NO.sub.x sensors, oxygen sensors, and residual
heat sensors, mounted on a common substrate for detecting specific gases
and temperatures in the exhaust gases and generating signals based
thereon, the substrate being an oxygen-ion-conductive ceramic material and
the sensors elements including conductive patterns applied to the common
substrate, and a common analyzer connected to the sensor units for
analyzing the signals generated by the sensor elements.
Inventors:
|
Lundgren; Staffan (Hind.ang.s, SE);
Jobson; Edward (Romelanda, SE);
Arlig; Ulf (K.ang.llered, SE);
Salomonsson; Per (Goteborg, SE);
Unosson; Anders (Goteborg, SE);
Hjortsberg; Ove (Goteborg, SE)
|
Assignee:
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AB Volvo (Goteborg, SE)
|
Appl. No.:
|
894391 |
Filed:
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September 8, 1997 |
PCT Filed:
|
February 21, 1996
|
PCT NO:
|
PCT/SE96/00235
|
371 Date:
|
September 8, 1997
|
102(e) Date:
|
September 8, 1997
|
PCT PUB.NO.:
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WO96/26434 |
PCT PUB. Date:
|
August 29, 1996 |
Foreign Application Priority Data
Current U.S. Class: |
204/426; 204/425; 205/781 |
Intern'l Class: |
G01N 027/407 |
Field of Search: |
204/421-429,412
205/781
|
References Cited
U.S. Patent Documents
3556950 | Jan., 1971 | Dahms | 204/419.
|
4452682 | Jun., 1984 | Takata et al. | 204/416.
|
4591422 | May., 1986 | Kato et al.
| |
4927517 | May., 1990 | Mizutani et al.
| |
5339627 | Aug., 1994 | Baier.
| |
5352353 | Oct., 1994 | Schonauer et al.
| |
5397442 | Mar., 1995 | Wachsman | 204/428.
|
5482609 | Jan., 1996 | Kobayashi et al. | 204/424.
|
5736028 | Apr., 1998 | Hjortsberg et al.
| |
Foreign Patent Documents |
35 04 498 C2 | Apr., 1989 | DE.
| |
42 28 052 A1 | Apr., 1993 | DE.
| |
7844330 | Jun., 1980 | GB.
| |
8700712 | Jul., 1987 | GB.
| |
92/14143 | Aug., 1992 | WO.
| |
94/00468 | Nov., 1994 | WO.
| |
Primary Examiner: Tung; T.
Attorney, Agent or Firm: Royston, Rayzor, Vickery, Novak & Druce, L.L.P.
Claims
What is claimed is:
1. Apparatus for analyzing exhaust gases from a combustion process
comprising at least one sensor unit mounted for direct contact with said
exhaust gases, said at least one sensor unit comprising a plurality of
sensor elements mounted on a common substrate for detecting specific gases
contained in said exhaust gases and generating signals based thereon,
wherein at least one of said sensor elements is a NO.sub.x sensor, said
NO.sub.x sensor including an oxygen-ion transporter, said NO.sub.x sensor
having an anode and a cathode, said cathode comprising a metal which is
selective for the dissociation of nitrogen oxide as compared to oxygen,
whereby said NO.sub.x sensor generates said signal based substantially
only on the NO.sub.x content of said exhaust gases and substantially
independent of the oxygen content of said exhaust gases, said substrate
comprising an oxygen-ion-conductive ceramic material and said plurality of
sensor elements comprising conductive patterns applied to said common
substrate, and a common analyzer unit connected to said at least one
sensor unit for analyzing said signals generated by said plurality of
sensor elements.
2. The apparatus of claim 1 wherein said plurality of sensor elements in
addition to the NO.sub.x sensor are selected from the group consisting of
lambda sensors, oxygen sensors, and residual heat sensors.
3. The apparatus of claim 2 wherein said NO.sub.x sensor comprises:
an external voltage source connected to said anode and said cathode, said
external voltage source being adapted to drive the current between said
anode and said cathode; and
measuring means for measuring said current connected to said anode and said
cathode, whereby the measurement of said current corresponds to a
measurement of the concentration of nitrogen oxides in the gas.
4. The apparatus of claim 3 wherein at least one of said anode and said
cathode comprises gold.
5. The apparatus of claim 2 wherein said residual heat sensor comprises a
first resistor including a surface exposed to said exhaust gases whereby
said first resistor increases its temperature-dependent resistance when
heated in the presence of oxidizable gas components in said exhaust gases,
and a second resistor, a measuring bridge connected to said residual heat
sensor, and a voltage source for supplying a voltage to said measuring
bridge.
6. The apparatus of claim 5 wherein said second resistor comprises a
reference for comparison with said resistance of said first resistor.
7. The apparatus of claim 2 wherein said plurality of sensor elements
includes an associated air gap in said common substrate, a first electrode
formed in said air gap, and a second electrode formed on said common
substrate exposed to said exhaust gases.
8. The apparatus of claim 2 wherein said plurality of sensor units
comprises a lambda sensor and a residual heat sensor, whereby said
apparatus can be used for diagnosis of a catalyzer for purification of
said exhaust gases.
9. The apparatus of claim 8 including an associated air gap within said
common substrate, and an electrode associated with said lambda sensor
arranged in said air gap.
10. The apparatus of claim 2 including a residual heat sensor whereby said
apparatus can be used for analyzing said exhaust gases from a diesel
engine.
11. The apparatus of claim 10 further comprising an oxygen sensor.
12. The apparatus of claim 1 wherein said common substrate comprises
stabilized zirconium dioxide or titanium oxide.
13. The apparatus of claim 1 including heating means for heating said
common substrate.
14. The apparatus of claim 13 wherein said heating means comprises a
heating wire mounted in said substrate, and including a voltage source
connected to said heating wire.
15. The apparatus of claim 1 including a protective cap substantially
covering said at least one sensor unit.
Description
FIELD OF THE INVENTION
The present invention relates to an arrangement for analyzing exhaust gases
from a combustion process.
BACKGROUND OF THE INVENTION
In the field of motor vehicle combustion engines, there is a desire to have
the ability to detect the concentration of different gaseous components in
the exhaust gas stream from the engine. Such measurements can be used for
controlling the operation of a combustion engine, with a view toward
optimizing the amounts of injected fuel and air. If the engine can be
provided with an optimal composition of the fuel/air mixture during all
operating conditions, the fuel consumption and the harmful emissions from
the combustion engine can be minimized.
In addition to engine control, such gas measurement should also provide the
ability to be used in connection with a diagnosis of a vehicle's catalytic
converter (catalyzer). In this context, the fuel and oxygen levels must
lie within certain ranges in order that the vehicle's catalyzer should be
able to operate optimally. A measure of the catalyzer's so-called
"light-off" time, i.e. the time which elapses before the catalyzer
purifies the exhaust gases optimally, can also be used during a diagnosis
of the catalyzer's operation.
Different forms of gas sensors are known for achieving the above-mentioned
objectives. One example of such a gas sensor, which is particularly for
use in connection with motor vehicles, is the so-called lambda sensor, by
means of which the oxygen content in the exhaust gases can be detected.
The signal from a lambda sensor can be used in connection with optimizing
the fuel and oxygen supply to the engine. In addition to the oxygen, it
would be desirable to detect other components in the exhaust gases.
Examples of known sensors (apart from lambda sensors) are thermistors,
NO.sub.x sensors (i.e. sensors for nitrogen oxide compounds), oxygen
sensors, carbon monoxide sensors and residual heat sensors.
An arrangement for detecting combustible gaseous hydrocarbons by means of a
measurement bridge which has pellistors (pellet resistors) is known from
British Patent No. 2,185,579. A pellistor is a resistor with a
temperature-dependent resistance, as described in British Patent No.
2,044,937, for example. An application of pellistors in connection with
the detection of exhaust gases in motor vehicles is described in Swedish
Patent Application No. 9301715-0.
In connection with the measuring and detecting of different gas components
in the exhaust gas stream from a combustion engine, a problem exists in
that measurement signals from certain of the above-mentioned sensors can
be influenced by other gases than those for which the sensor is intended.
For example, the NO.sub.x sensor (apart from sensing the concentration of
nitrogen oxide compounds) can also be sensitive to the concentration of
oxygen and hydrocarbons. By using a plurality of different types of
sensors at the same time, it should thus be possible to separate out each
of the different gas components and, despite the cross-sensitivity of the
different sensors, obtain a measurement of the composition of the measured
gas. Using a plurality of different sensors in this way is, however,
expensive and requires space. Each separate gas sensor requires a probe, a
fixture, cabling, possibly an amplifier, an analyzer unit and a common
analyzer unit which, from the signals of the different sensors, produces
output signals giving the composition of the measured gas.
SUMMARY OF THE INVENTION
In accordance with the present invention, these and other objects have now
been realized by the invention of apparatus for analyzing exhaust gases
from a combustion process which comprises at least one sensor unit mounted
for direct contact with the exhaust gases, the at least one sensor unit
comprising a plurality of sensor elements mounted on a common substrate
for detecting specific gases contained in the exhaust gases and generating
signals based thereon, the substrate comprising an oxygen-ion-conductive
ceramic material and the plurality of sensor elements comprising
conductive patterns applied to the common substrate, and a common analyzer
unit connected to the at least one sensor unit for analyzing the signals
generated by the plurality of sensor elements. Preferably, the plurality
of sensor elements include lambda sensors, NO.sub.x sensors, oxygen
sensors, and residual heat sensors.
In accordance with one embodiment of the apparatus of the present
invention, the common substrate comprises stabilized zirconium dioxide or
titanium oxide. In one preferred embodiment, the NO.sub.x sensor includes
oxygen-ion-transport means whereby the NO.sub.x sensor generates the
signal based substantially only on the NO.sub.x content of the exhaust
gases and substantially independent of the oxygen content of the exhaust
gases. In a preferred embodiment, the oxygen-ion-transport means comprises
the conductive pattern including a first conductive pattern comprising an
anode, a second conductive pattern comprising a cathode, an external
voltage source for driving a current comprising the oxygen ions, and
current measuring means for measuring the current so as to provide the
signal as a measure of the concentration of the NOx-compounds in the
exhaust gases. Preferably, at least one of the anode and the cathode
comprises gold.
In accordance with another embodiment of the apparatus of the present
invention, the residual heat sensor comprises a first resistor including a
surface exposed to the exhaust gases whereby the first resistor increases
its temperature dependent resistance when heated in the presence of
oxidizable gas components in the exhaust gases, and the second resistor, a
measuring bridge connected to the residual heat sensor, and a voltage
source for supplying a voltage to the measuring bridge. Preferably, the
second resistor comprises a reference for comparison with the resistance
of the first resistor.
In accordance with another embodiment of the apparatus of the present
invention, the apparatus includes heating means for heating the common
substrate. Preferably, the heating means comprises a heating wire mounted
in the substrate, and including a voltage source connected to the heating
wire.
In accordance with another embodiment of the apparatus of the present
invention, the apparatus includes a protective cap substantially covering
the at least one sensor unit.
In accordance with another embodiment of the apparatus of the present
invention, at least one of the lambda sensors, NO.sub.x sensors and oxygen
sensors comprises at least one of the plurality of sensor units, and the
at least one of the lambda sensor, the NO.sub.x sensor and the oxygen
sensor includes an associated air gap in the common substrate, a first
electrode formed in the air gap, and a second electrode formed on the
common substrate exposed to the exhaust gases.
In accordance with another embodiment of the apparatus of the present
invention, the plurality of sensor units comprises a lambda sensor and a
residual heat sensor, whereby the apparatus can be used for diagnosis of a
catalyzer for purification of the exhaust gases. Preferably, the apparatus
includes an associated air gap within the common substrate, and an
electrode associated with the lambda sensor arranged in the air gap.
In accordance with another embodiment of the apparatus of the present
invention, the plurality of sensor units includes at least one NO.sub.x
sensor and a residual heat sensor whereby the apparatus can be used for
analyzing the exhaust gases from a diesel engine. Preferably, the
apparatus further comprises an oxygen sensor.
It has thereby been found that by placing a number of these sensors on one
or more common substrates and by arranging the sensor elements in an
integrated "multisensor" with a single attachment fixture, as well as
common cabling, amplifier unit and analyzer unit, an analysis of the
different gas components in the gas mixture present is possible.
BRIEF DESCRIPTION OF THE DRAWINGS
The invention may be more fully appreciated with reference to the following
detailed description, which, in turn, refers to the Figures in which:
FIG. 1 is a schematic representation of a measuring system for use in
accordance with the present invention;
FIG. 2 is a top, perspective, partially schematic view of a sensor unit in
accordance with the present invention;
FIG. 3 is a side, sectional view of a lambda sensor used in accordance with
the present invention;
FIG. 4 is a top view of an NO.sub.x sensor used in accordance with the
present invention;
FIG. 5 is top view of an oxygen sensor used in accordance with the present
invention;
FIG. 6 is a top view of a residual heat sensor used in accordance with the
present invention;
FIG. 7 is a schematic representation of Wheatstone-bridge intended to be
used in connection with a residual heat sensor as shown in FIG. 6;
FIG. 8 is a top view of a sensor unit in accordance with one embodiment of
the present invention for use in connection with diesel engines;
FIG. 9 is side, sectional view of the sensor unit shown in FIG. 8;
FIG. 10 is a top view of another embodiment of a sensor unit in accordance
with the present invention;
FIG. 11 is a side, sectional view of the sensor unit shown in FIG. 10; and
FIG. 12 is a partial, side, partially sectional view of a plurality of
sensor units provided with a protective cap in accordance with the present
invention.
DETAILED DESCRIPTION
Referring to the drawings, in which like reference numerals refer to like
elements thereof, FIG. 1 schematically shows an arrangement which includes
the present invention. According to a preferred embodiment, the
arrangement comprises a sensor unit 1 consisting of a plurality of
sensors, i.e. an integrated "multisensor," which is intended to be placed
in the exhaust outlet of a motor vehicle. In accordance with the
description which follows, the sensor unit 1 can comprise sensors for
detecting NOx-compounds (nitrogen oxide compounds) and oxygen. The sensor
unit 1 can further comprise a residual heat sensor which is composed of a
known pellistor, a temperature sensor and a lambda sensor. Each separate
sensor which is included in the sensor unit 1 emits a signal X.sub.i,
where i=1, 2, . . . n.
The signals X.sub.i supplied from the sensor unit 1 are supplied by cabling
2 to a measuring unit in the form of a filter- and amplifier-unit 3 which
comprises filter and amplifier circuits for treatment of the respective
signals X.sub.i. The treated signal packet X is supplied to an analyzer
unit 5 by means of second cabling 4, unit 5 preferably being
computer-based, in order to produce a measurement of the temperature and
of the amounts of the gas components which are detected by the sensors in
the sensor unit 1. The analyzer unit can be made according to the
principle of template-recognition (pattern-recognition), e.g. of the
neuro-net type. The signal packet X from the sensor unit 1, the signals
X.sub.i constitute a resolvable combination of the size of the different
gas components. With the aid of suitably-chosen algorithms, the analyzer
unit 5 can break down the signal packet X into its components Y.sub.i from
the respective signals X.sub.i of the sensor unit 1.
A number of measurement signals Y.sub.i is supplied from the analyzer unit
5, said signals providing a measure of the different substances which have
been detected by the sensor unit 1, e.g. concerning the concentration of
oxygen and NO.sub.x -compounds and the temperature. These signals are then
supplied to the vehicle's control system and are used for controlling the
operation of the engine as well as for diagnosis of the catalyzer's
operation.
FIG. 2 shows a detailed view of a sensor unit 1 which is intended to be
placed in the gas stream in the exhaust system of a motor vehicle. The
sensor unit 1 comprises a substrate 7 which is common for all of the
included sensors. The substrate 7 comprises, in accordance with this
embodiment, oxygen-ion-conductive zirconium-dioxide, ZrO.sub.2, which is
stabilized, i.e. "fixed" in a certain crystal structure which is
advantageous with respect to the conductivity for oxygen ions.
Yttrium-oxide can preferably be used as a stabilizer. A lambda probe 8, an
NO.sub.x -sensor 9, an oxygen sensor 10 and a residual heat sensor 11 are
arranged on the substrate 7. A voltage-measurement unit 12 is combined
with the lambda sensor 8. A voltage source 13 and a current measurement
unit 14 are combined with the NO.sub.x -sensor 9. A further voltage source
15 and a further current-measuring unit 16 are associated with the oxygen
sensor 10. The residual heat sensor 11 is connected to a measuring bridge,
as will be described in detail below.
The voltage and current measuring units 12, 14, and 16, shown in FIG. 2 are
only shown schematically and are included in the filter- and
amplifier-circuit 3 (see FIG. 1) which thus constitutes a common measuring
unit supplying the signal packet X to the analyzer unit 5.
FIG. 3 shows a cross-sectional view through the lambda probe 8. On the
upper side of the substrate 7 there is a first electrode 17 which is
composed of platinum. A second electrode 18, which functions as a
reference electrode, is arranged within an air gap 19 which extends
through the lambda probe 8. Additionally, there is a heating element 20
incorporated into the substrate 7. The heating element 20 is composed of
an electrode, preferably of platinum or tungsten, which is connected to an
external voltage source (not shown). The substrate 7 can be heated up to a
correct working temperature (400.degree. C.-800.degree. C.) with the aid
of the heating element 20. The potential difference between the two
electrodes, 17 and 18, can be measured by connection (not shown) to the
above-mentioned voltage measuring unit 12. The potential difference
constitutes a measure of the lambda-ratio (i.e. rich or lean) of the gas
which surrounds the lambda sensor 8.
FIG. 4 shows a view from above of the NO.sub.x -sensor 9 which comprises
two electrodes, 21 and 22, respectively, which constitute the cathode and
anode, respectively. The electrodes, 21 and 22, or at least the cathode,
are made of gold, in accordance with this embodiment. When the NO.sub.x
-sensor 9 is surrounded by a gas which contains NO.sub.x -compounds, these
will be adsorbed on the surface of the sensor 9, i.e. on the electrodes,
21 and 22, and the substrate 7. A selective dissociation, i.e. a
decomposition, will occur thereafter so that negative oxygen ions,
O.sup.-, are formed at the cathode 21. With the assistance of the voltage
applied by the voltage source 13 (see FIG. 1), the oxygen ions can be
transported through the oxygen-ion-conductive substrate. Molecular oxygen,
O.sub.2, is formed at the anode 22, which oxygen desorbs from the NO.sub.x
-sensor's 9 surface back into the gas phase. At the same time as the
oxygen atoms are ionized at the cathode 21, the nitrogen atoms recombine
into molecular nitrogen, N.sub.2, and return from the surface of the
NO.sub.x -sensor 9 into the gas phase.
In accordance with a further embodiment of the NO.sub.x -sensor 9, only the
anode 22 is made of gold. In this case the cathode 21 is made of platinum,
for example.
With the aid of the above-mentioned current measuring unit 14 (see FIG. 1),
the oxygen-ion current occurring in the circuit can be measured. This
measured current thus constitutes a measure of the amount of NO.sub.x
-compounds in the gas stream.
Measuring with the NO.sub.x -sensor 9 is selective, i.e. the oxygen ion
current which occurs in the sensor 9 originates mainly from the NO.sub.x
-compounds included in the gas stream. The measuring of the NO.sub.x
-compounds in the NO.sub.x -sensor 9 is thus substantially independent of
the concentration of oxygen in the gas stream.
The selective function of the NO.sub.x -sensor 9 is obtained by the
formation of the substrate 7, which is oxygen-ion-conductive, and of the
electrodes, 21 and 22, of which at least one is gold. Furthermore, the
selectivity can be affected by the choice of pump voltage, i.e. with the
aid of the voltage applied by the voltage source 13. The invention is
therefore particularly suitable for measurements of NO.sub.x -compounds in
connection with exhaust gases in which the oxygen content varies, and
gives a measurement which is substantially independent of the variations
in the oxygen concentration of the exhaust gases.
During transport from the cathode 21 to the anode, the oxygen ions will
primarily be displaced along the outer layer of the substrate 7. This
provides a good time response during measurement with the NO.sub.x -sensor
9.
In order that the transport of the oxygen ions occurs in an optimal manner,
the respective electrodes, 21 and 22, are formed as a straight line with a
number of transverse lines arranged so that they project substantially
perpendicularly from the straight line. The two conductive patterns are
arranged so that they "project into one another." This arrangement means
that the interface between the electrodes, 21 and 22, the substrate 7, and
the gas in which the sensor 9 is located, is made as large as possible. In
this manner, transport of the negative oxygen ions can be maximized, which
contributes to a high current through the NO.sub.x -sensor 9.
Additionally, it is most important that the distance between the
electrodes 21 and 22 is as small as possible, which yields a short
response time during measurements with the NO.sub.x -sensor 9.
FIG. 5 shows a view from above the oxygen sensor 10. A conductive pattern
is arranged on the substrate 7, said pattern being in the form of two
electrodes, 23 and 24, respectively. In the same way as the electrodes 21
and 22 of the aforementioned NO.sub.x -sensor 9, the electrodes 23 and 24
of the oxygen sensor 10 are formed as a straight line with a plurality of
transverse lines which project substantially perpendicularly from the
straight line. The electrodes, 23 and 24, are preferably of platinum. By
applying a voltage over the electrodes, 23 and 24 (with the aid of the
voltage source 15 shown in FIG. 1), a current moves in the circuit in the
presence of oxygen. This occurs due to the substrate 7 being conductive
for oxygen ions at high temperatures (400.degree. C.-800.degree. C.). This
oxygen-ion current can be measured with the aid of the current measuring
unit 16 shown in FIG. 1. The size of the measured current is proportional
to the oxygen concentration in the gas surrounding the oxygen sensor 10.
In addition to oxygen, the oxygen sensor 10 is influenced by, for example,
NO.sub.x -compounds, hydrocarbons and hydrogen.
FIG. 6 shows the residual heat sensor 11 which is based on a so-called
pellistor which is a type of sensor known (per se) from Swedish Patent
Application No. 9301715-0. The residual heat sensor 11 comprises a
conductive pattern 25 which forms two resistors, a first resistor AC which
is formed by the conductive pattern between the points A and C, and a
second resistor BC which is formed by the conductive pattern between the
points B and C. The conductive pattern 25 is composed of platinum, and
both of the resistors, AC and BC, have the same resistance at the same
temperature.
The resistance of the resistors AC and BC increases linearly with
temperature. The first resistor AC is coated with a passive layer,
preferably AL.sub.2 O.sub.3, which is gas-tight, i.e. the surface of the
conductive pattern cannot be influenced by the surrounding gas. The second
resistor BC is coated with a catalytically active wash-coat 26. The
hydrocarbons and the carbon monoxide will be burned on the active
wash-coat 26 in the presence of oxygen. This combustion brings about a
temperature increase in the first resistor BC which means that its
resistance increases somewhat with respect to the resistance of the
resistor AC. By coupling the resistances AC and BC in a so-called
Wheatstone-bridge which is shown in FIG. 7, the small resistance changes
which result from the combustion of the oxidizable substances on the
wash-coat 26 can be detected. The denotations A, B, and C in FIG. 7
correspond to what is shown in FIG. 6.
The second resistance AC functions as a reference which is subjected to the
same environment (ambient temperature, flow, air humidity, etc.) as the
first resistor BC. This means that only the resistance change resulting
from the combustion heat produces a resistance difference between the two
resistors. With the aid of a voltage measuring unit 27, the voltage over
the Wheatstone-bridge can be measured. This voltage is proportional to the
residual heat in the gas, i.e. the amount of unburnt oxidizable substances
in the gas.
The present invention can be used in applications having different exhaust
gas compositions. In those cases where rich mixtures occur, the sensor
unit 1 can be arranged in such a way that an electric voltage is connected
over the ceramic oxygen-ion-conductive substrate, whereby one side of the
substrate has access to the atmosphere and the other side of the substrate
has access to the gas which is to be analyzed. In this manner, the
necessary oxygen for complete combustion of the hydrocarbons and the
carbon monoxide is supplied to the catalytically active wash-coat 26. In
turn, this makes the measurements substantially independent of the oxygen
content in the exhaust gases.
A particular application of the sensor unit according to the present
invention is as a sensor for diesel exhaust gases. Such exhaust gases
contain between about 5 and 20% oxygen, soot, nitrogen-oxides,
carbon-oxides and hydrocarbons. FIG. 8 shows a view from above of a sensor
unit 38 according to the present invention which comprises a NO.sub.x
-sensor 9, an oxygen sensor 10 and a residual heat sensor 11. FIG. 9 shows
a side view of the same sensor unit 28. The NO.sub.x -sensor 9 comprises,
as described above, two electrodes, 21 and 22, of which at least one is
gold. The oxygen sensor 10 comprises two electrodes, 23 and 24, of
platinum. The residual heat sensor 11 comprises a conductive pattern 25 of
platinum with a layer 26 of wash-coat and a passivating layer. As shown in
FIG. 9, the sensor unit 28 comprises the heating element 29.
In accordance with a possible embodiment for analysis of diesel exhaust
gases, the invention may include only one NO.sub.x -sensor and a residual
heat sensor, i.e. no oxygen sensor. This is possible in particular with
heavy diesel vehicles having combustion engines, to which a predetermined
amount of fuel and air is injected at a specific operating condition.
Since the amount of fuel and air is known, the amount of oxygen in the
exhaust gases can be determined with sufficiently high accuracy. In this
case, this results in no separate oxygen sensor being necessary.
The sensor unit 28 can be used in order to determine the NO.sub.x and
oxygen concentration and the amount of residual heat in the form of
hydrocarbons and carbon monoxide in the exhaust gases. These signals can
be used, for example, to alter the control of the diesel engine so as to
reduce the emissions from the engine.
In accordance with a further application of the present invention, this can
be used in connection with diagnosis of a three-way exhaust catalyzer.
Vehicles which are equipped with such a catalyzer must have an exhaust gas
composition which is stoichiometric (i.e. lambda=1) for optimal conversion
of the three exhaust gas components NO.sub.x, CO and HC. With unknown
requirements for cleaner cars, the catalyzer's effectiveness must be able
to be diagnosed in the vehicle during operation (so-called "onboard
diagnosis"). A sensor unit 30 in accordance with the present invention,
which is shown in FIGS. 10 and 11 can, for this purpose, comprise a
residual heat sensor 11 and a lambda sensor 8. The sensor unit 30 can be
used both for regulating the engine control in order to achieve a maximum
conversion of the three exhaust gas components and for diagnosing the
catalyzer's effectiveness and absolute exhaust gas levels. The sensor unit
30 comprises a lambda sensor 8 with electrodes 17 and 18 and a residual
heat sensor with a conductive pattern 25 as well as a wash-coat layer 26.
The sensor unit 30 further comprises an air gap 19 and a heating element
20.
The sensor units 28 and 30 as shown in FIGS. 8-11 are connected to a
filter- and amplifier-circuit and an analyzer circuit of the same type as
mentioned above in connection with FIGS. 1 and 2.
The sensor units 1, 28 and 30 as described above are intended to be placed
in the exhaust gas outlet of a motor vehicle. Instead of using only one
sensor unit, a plurality of different sensor units can also be used, which
can then be grouped together. The different sensor units can comprise
different constellations of sensors which preferably constitute NO.sub.x
-sensors, lambda sensors, oxygen sensors and residual heat sensors. FIG.
12 shows such a group of sensor units, which in this case comprises three
different sensor units 31, 32 and 33. The sensor units 31, 32 and 33 are
joined with a measuring unit in the form of a filter- and amplifier-unit
(see FIG. 1) by means of a common cable 34. The sensor units 31, 32 and 33
are arranged in the exhaust system 35 of a motor vehicle. The flow
direction of the exhaust gases is indicated by arrow 36. The sensor units
31, 32 and 33 are preferably provided with a protective cap 37 which
reduces the cooling effect which can be caused by the flowing exhaust
gases, which means that a high and even temperature is obtained within the
protective cap 37. The protective cap 37 is provided with at least one
hole 38, or alternatively a slit or the like, so that the sensor units 31,
32 and 33 will be exposed to the exhaust gases. The hole 38 can be
arranged in different ways, e.g. in the top of the protective cap 37.
According to a possible variation of the present invention, it can be
provided with a so-called linear lambda sensor which is a sensor emitting
a signal proportional to the oxygen concentration in the surrounding gas.
The signal which is emitted is proportional to the oxygen concentration on
the lean side and the rich side of .lambda.=1. Such a linear lambda sensor
can be arranged as a replacement for the above-mentioned oxygen sensor.
With the arrangement according to the present invention, a number of
advantages are obtained. Firstly, a more exact value of, for example,
NO.sub.x -concentrations can be obtained if the values of HC and CO are
known at the same time. Additionally, all of the sensors are subject to
the same temperature if they are arranged at the same point. Furthermore,
the combination of one or more sensors at one and the same point allows a
system analysis by means of template-recognition of the neuro-net type.
For example, in the regulation of transients in an engine's operation it is
important that the parameters are measured in the same time-window in the
combustion process, which is achieved by the sensors being gathered at one
and the same point. Furthermore, if the sensors are at the same point, the
problems with calibration can be avoided, which otherwise could occur with
sensors placed at different locations where the temperature and the gas
composition are not the same.
An additional advantage is present in that the sensor unit according to the
present invention only requires one attachment fixture, one cable, etc.
Furthermore, an advantage is obtained in that the sensor unit uses a
common, oxygen-ion-conductive substrate.
In addition, in accordance with alternative embodiments, the oxygen sensor
10 as well as the NO.sub.x -sensor 9 can be formed with an air gap of
similar type to the air gap 19 described above in connection with FIG. 3.
This air gap functions as a reference chamber in which a first electrode,
i.e. a reference electrode, is placed. A second electrode is arranged on
the substrate and is subjected to the gas which is the object of
measurement.
Furthermore, the oxygen sensor's 10 electrodes, 23 and 24, can be arranged
on respective sides of the substrate 7. This is also valid for the
NO.sub.x -sensor's 9 electrodes 21 and 22.
Although the invention herein has been described with reference to
particular embodiments, it is to be understood that these embodiments are
merely illustrative of the principles and applications of the present
invention. It is therefore to be understood that numerous modifications
may be made to the illustrative embodiments and that other arrangements
may be devised without departing from the spirit and scope of the present
invention as defined by the appended claims.
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